Earthing cable tray

[Revved Up Sparky]

I work as an electrical inspector for a large electrical contracting company and regularly follow after installation teams, checking their work and producing EICs.

At present I am inspecting, testing and certifying a new installation at a new school, a large one which is due to be handed over in a few weeks.

It seems the council have chosen the cheap option, rather than full containment the school has been wired using twin and earth fixed to cable tray.

There are sections of cable tray above ceilings all over the place, loaded with dozens of twin and earth cables neatly tie wrapped yet none of these cable trays have been earthed to the MET.

I wouldn't be quite so concerned if these cable trays were carrying data or SWA cables. But in my mind these twin and earths are vulnerable to damage, for example, if they get struck with an object, which could result in a live conductor suffering insulation damage, touching the tray and making it live.

What motivates you to earth cable tray ? do you look at what is on the tray before you earth it ? do you always earth cable tray regardless of what is on it ? or do you say to yourself " is there a reasonable possibility that this tray could become live ?" and if the answer is yes you earth it.

If you were in my shoes would you call the installers back to fit earths to each section of tray carrying twin and earths ?

I'd be grateful to hear your thoughts.

Many thanks

RUS

 

[binky]

extraneous conductive part, so should have an earth bond, though I would probably pick up an earth from a circuit rather than bond back to MET.

 

[Risteard]

extraneous conductive part

Extremely unlikely.

 

[Rob_the_rich]

Are all circuits 30mA RCD protected?

loaded with dozens of twin and earth cables neatly tie wrapped

Carrying full load? grouping factor?

 

[Revved Up Sparky]

extraneous conductive part, so should have an earth bond, though I would probably pick up an earth from a circuit rather than bond back to MET.

Thanks for the reply. Many trays are suspended by studded bars & unistrut fixed to a concrete ceiling whilst others are attached to the structural steel which is bonded to earth therefore I do not think that the trays are introducing a different earth potential.

 

[binky]

think of it as being like metal back boxes, albeit the cable in theory is still Class II insulated unless the outer sheath has been stripped.

 

[binky]

from certain registration body, seems I am wrong under certain criteria:-

Normally not, unless there are specific circumstances, such as:
 

the installation designer has selected the cable tray or basket for use as a protective conductor as permitted by Regulation 543.2.1(vi), in which case the requirements of Regulation 543.2.5 relating to continuity and cross-sectional area would need to be met, and the tray or basket would need to be earthed, or

the circumstances are such that cable tray or basket meets the definition of an extraneous-conductive-part, then main protective bonding of the tray or basket will be required.

A cable tray, ladder rack or basket system designed to be used as a protective conductor in an electrical installation will be classified as having electrical continuity characteristics according to clause 6.3.2 of BS EN 61537: 2007 Cable management – Cable tray systems and cable ladder systems.
 
In practice, unless there are exceptional circumstances such as those indicated above, the conductive parts of a cable tray or basket will neither be exposed-conductive-parts nor extraneous-conductive-parts. Consequentially, neither cable tray nor cable basket need be intentionally earthed or bonded. The explanation is given below.

Turning first to earthing 
Cables installed on a metal cable tray or basket will normally either have exposed-conductive-parts, such as metallic armour, or have a non-metallic sheath or non-metallic enclosure.
 
Exposed-conductive-parts of a cable are required to be connected to the Main Earthing Terminal of the installation by a circuit protective conductor. The cable tray or basket on which the cable is installed is not itself an exposed-conductive-part and hence is not required to be earthed.
 
A cable that is rated at not less than 300/500 V and has a non-metallic sheath or a non-metallic enclosure is considered to meet the requirements of Regulation 412.2 for both basic protection and fault protection for the protective measure double or reinforced insulation, provided the wiring system is in accordance with Chapter 52 (Regulation 412.2.4.1 refers). Hence a metal cable tray or basket on which the non-metallic sheathed cable is installed is not required be earthed.
 
Now considering bonding 
Unless a metal cable tray or basket will introduce a potential, generally Earth potential, that does not already exist in the location, it will not meet the definition of an extraneous-conductive-part. In normal circumstances, therefore, the tray or basket need not be connected to a main bonding conductor or a supplementary bonding conductor.
 
An example of where, exceptionally, a metal cable tray or basket would require main bonding is where it carries services into a building and is in contact with the ground outside the building.
- See more at: https://www.niceic.com/contractor/services/technical-library/does-bs-7671-require-metal-cable-tray-or-basket-to#sthash.lkuG5bVg.dpuf

 

[Tony S]

A nice bit of detective work there Binky.

 

[Revved Up Sparky]

+1 and Binky scoobed. Thanks for taking the time to research and type Binky, I appreciate it.

If I was installing, I think I would still want to earth a cable tray carrying twin and earth - not a requirement maybe but good practice IMHO. To me there is a reasonable risk -depending on the environment in which the tray is sited -  of one of the twin & earth cables getting clouted by something thus damaging the insulation and perhaps a live core touching the tray and making it live. 

 

[revjames]

I always earth cable tray whatever its carrying.

 

[Tony S]

I always earth cable tray whatever its carrying.

Why?

 

[Revved Up Sparky]

Because he is a very down to earth fellow ! 

 

[revjames]

Why?

BECAUSE ITS GOOD PRACTICE

 

[Tony S]

There’s no need to SHOUT!

Good practice should have a logical reason, otherwise it’s a waste of time and materials.

 

[Blue Duck]

There’s no need to SHOUT!

Good practice should have a logical reason, otherwise it’s a waste of time and materials.

Or Under engineering. ..

The cheap solution: 

http://www.canford.co.uk/PLASTIC-CABLE-TRAY

 

[Andy™]

but what about cable support in case of fire?

 

[Tony S]

Pass on that one, although it would be interesting so see what would happen if it was installed near to a kiln.

 

[Lurch]

but what about cable support in case of fire?

Just support it every 300mm with some Unistrut. Simple.